A recent study published in the journal Biology Letters describes an unruly scene stumbled across by Dr Reuben Shipway from the University of Portsmouth when studying clams. The focus species, also known as shipworms, are a gender-fluid, worm-like animal that is infamous for wreaking havoc on ships, piers, and docks as it munches through the wood. Here, it sits with two body parts protruding, one for feeding and the other for getting rid of waste. Shipway was monitoring the reproductive habits of shipworms when one day he saw something unexpected.
"When we first noticed these animals reproducing in the aquarium, we couldn't believe what we were seeing,” he said in a statement. “They were using their siphons to wrestle and inseminate one another and trade sperm. As far as we know, this wrestling and jousting hasn't been reported before."
The competitive sexual frenzy unfolding before his eyes saw the clams competing for who had the most impressive appendages for hours at a time. This kind of sparring between individuals is a novel observation for shipworm copulation, revealing new insights into the reproductive habits of these marine pests.
The indecent footage reveals a string of worms exchanging sperm, with some individuals being on the receiving end of sperm at one end of a siphon while giving their own sperm to someone else using the other. There are also some worms removing competitor’s sperm while they wrestle their way towards replacing it with their own. The orgy involved 74 of the 79 shipworms stuffed inside a single piece of wood, the remaining five of which were simply too far away to get involved.
The researchers were able to review up close the process that unfolds during shipworm copulation. First of all, shipworms looking to inseminate another will march across some wood until they find a willing recipient siphon. They then clamp down and entwine with the receiver, exchanging sperm in the process. In areas of high competition, such as our 74-worm sex frenzy, siphons will wrestle for the chance to mate. Once copulation is successfully completed, the fertilized eggs are released into the ocean.
"It's a rare and sophisticated form of reproductive behaviour, with sparring between rival mates, pulling potential mates closer and away from rivals, and going as far as to include pulling a rival's sperm out of a siphon so it floats away,” said Shipway. "Shipworms have evolved a stunning diversity of reproductive strategies, some simply spawn their eggs and sperm into the water, some recruit a harem of dwarf males to mate with, and now we know they compete to directly inseminate each other using their siphons."